whatever.js

class Whatever extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      a: 5 + props.b;
    };
    this.offset = 0;
    this.someEvent = this.someEvent.bind(this);
  }
  componentWillReceiveProps(nextProps) {
    this.setState({
      a: this.state.a + this.props.b + this.offset
    });
  }
  someEvent(evt) {
    this.offset += evt.clientX;
  }
  render() {
    return <DoesntMatter/>
  }
}

Suggest something more like:

class Whatever extends React.Component {
  state = {
    a: 5 + this.props.b,
    offset: 0
  };

  static getDerivedStateFromProps(nextProps, prevState) {
    return {
      a: prevState.a + nextProps.b + prevState.offset
    };
  }

  someEvent = ({ clientX }) =>
    this.setState(prevState => ({
      offset: prevState.offset + clientX
    }));

  render() {
    return <DoesntMatter />;
  }
}